Adsorption of heavy metal ions from aqueous solution by polyrhodanine-encapsulated magnetic nanoparticles

Adsorption of heavy metal ions from aqueous solution by polyrhodanine-encapsulated magnetic nanoparticles

Journal of Colloid and Interface Science 359 (2011) 505–511 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

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Journal of Colloid and Interface Science 359 (2011) 505–511

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Adsorption of heavy metal ions from aqueous solution by polyrhodanine-encapsulated magnetic nanoparticles Jooyoung Song, Hyeyoung Kong, Jyongsik Jang ⇑ WCU Program of Chemical Convergence for Energy and Environment (C2E2), School of Chemical and Biological Engineering, College of Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea

a r t i c l e

i n f o

Article history: Received 5 February 2011 Accepted 11 April 2011 Available online 18 April 2011 Keywords: Heavy metal ion Heavy metal adsorption Polyrhodanine Magnetic nanoparticles Maghemite

a b s t r a c t Polyrhodanine-coated c-Fe2O3 nanoparticles, synthesized by one-step chemical oxidation polymerization, were applied to the process of removal of heavy metal ions from aqueous solution. Factors influencing the uptake of heavy metal ions such as solution pH, initial metal ion concentration, contact time, and species of metal ions were investigated systematically by batch experiments. The adsorption equilibrium study exhibited that the Hg(II) ion adsorption of polyrhodanine-coated magnetic nanoparticles followed a Freundlich isotherm model than a Langmuir model. The kinetic data of adsorption of Hg(II) ion on the synthesized adsorbents were best described by a pseudo-second-order equation, indicating their chemical adsorption. In addition, the synthesized nano-adsorbents can be repeatedly used with help of an external magnetic field due to their magnetic properties. This work demonstrates that the magnetic polyrhodanine nanoparticles can be considered as a potential recyclable adsorbent for hazardous metal ions from wastewater. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Water pollution by heavy metal ions has become a serious environmental issue especially due to their toxicity and tendency to bioaccumulation [1,2]. The heavy metal ions are not only toxic to living organisms in water, but also cause harmful effects to land animals including humans through food chain transfers. In living organisms, heavy metal ions can particularly bind to nucleic acids, proteins, and small metabolites. The contaminated organic cells are altered or missed their biological functions with losing the homeostatic control of essential metals, resulting in fatal health problems [3,4]. Therefore, it is necessary to eliminate such hazardous heavy metal ion in wastewater before discharging it into the ecosystem. Several techniques, such as ion exchange [5], chemical precipitation [6], membrane processes [7,8], electro-dialysis [9], and adsorption [10–12] have been developed for the removal of heavy metals from aqueous media. Among these purification methods, the adsorption process using adequate adsorbents is considered as one of the most efficient and economical techniques in the viewpoint of simple design and facile handling. Numerous efforts have been contributed to the development of effective adsorbents like activated carbon [13], chitosan [4,14], zeolite [15], polymer [16,17], functionalized silica [18–21], and clay [22–25]. Besides, ⇑ Corresponding author. Tel.: +82 2 880 7069; fax: +82 2 888 1604. E-mail address: [email protected] (J. Jang). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.04.034

there is also a huge interest to adsorption on metallic and coated surfaces in the field of petroleum science [26–28]. Polyrhodanine has attracted considerable attention in various application fields such as anticorrosion [29], antibacterial [30,31], and antihistaminic agents [32]. In addition, they can be used for detecting or adsorbing of metal ions because the rhodanine monomeric unit has a metal-binding functional groups [33–37]. According to the hard and soft acids and bases (HSAB) theory introduced by Pearson, oxygen, nitrogen, and sulfur atoms are regarded as metal-binding atoms with strong affinity for heavy metal ions like lead, cadmium, mercury, etc. [38]. Polyrhodanine can be expected as a promising candidate for efficient adsorbent of heavy metal ions because it contains oxygen, nitrogen, and sulfur atoms in its monomeric structure. We previously reported the facile one-step chemical oxidation polymerization for the preparation of magnetic c-Fe2O3/polyrhodanine nanoparticles [39]. In the present work, the synthesized magnetic polyrhodanine nanoparticles were evaluated in their heavy metal ions adsorption performances. The various papers related to the heavy metal removal using magnetic nano- or microparticles have been reported [40–45]. It is anticipated that the polyrhodanine-encapsulated magnetic nanoparticles (PR-MNPs) prepared under one-pot synthesis system have some advantages for heavy metal removal such as easy recovery by an external magnetic field and excellent metal-binding activity due to its magnetic property and large surface area, respectively. The adsorption mechanism and kinetics of the PR-MNPs were systematically

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investigated with various experimental parameters such as solution pH, initial metal ion concentration, contact time, and species of metal ions. In order to study the adsorption behavior of the PR-MNPs, adsorption isotherm models and kinetic models were applied to the experimental data. The PR-MNPs could be easily recycled based on their magnetic properties; the recyclability of the PR-MNPs was also investigated.

concentration of metal ions before and after adsorption in mg/L, Ce is the equilibrium concentration of metal ions in mg/L, V is the volume of metal ions solution in liter scale, and W is the weight of the adsorbent in gram scale. The Ce values were defined as the remained Hg(II) ion concentration after 12 h of shaking time because it was determined that the adsorption of Hg(II) ions onto the PRMNPs sufficiently reached an equilibrium state for 12 h based on the experimental data.

2. Material and methods 2.4. Effect of contact time to adsorption capacity of Hg(II), Cd(II), Mn(II) and Cr(III) ions onto the PR-MNPs

2.1. Materials Rhodanine (97%), iron chloride (FeCl3) (97%), and sodium borohydride (NaBH4) (99%) were purchased from Aldrich (Milwaukee, WI) and used without further purification. For heavy metal adsorption properties, mercury nitrate monohydrate (Hg(NO3)2H2O) (98%), cadmium nitrate tetrahydrate (Cd(NO3)24H2O) (98%), manganese sulfate monohydrate (MnSO4H2O) (98%), and chromium nitrate nonahydrate (Cr(NO3)39H2O) (99%) were purchased from Aldrich (Milwaukee, WI). The hydrochloric acid (37%), nitric acid (70%), and ammonia solution (37%) were also bought from Aldrich (Milwaukee, WI) for control of solution pH. 2.2. Fabrication of polyrhodanine-encapsulated magnetic nanoparticles (PR-MNPs)

Typically, 5.0 mg of the PR-MNPs was added into 10 mL of Hg(II) ion solution (80 mg/L) at a pH value of 4.0. Then, the prepared samples were shaken at 350 rpm using a mechanical shaker at 25 °C. After a desired time, the PR-MNPs were removed from the solution using an external magnetic field. The residual Hg(II) ion concentration was determined by inductively coupled plasma (ICP) analysis. The analysis of the batch adsorption of the metal ion was carried out as a function of contact time (from 30 min to 12 h). The adsorption capacities of Cd(II), Mn(II), and Cr(III) ions were also determined similarly. For accurate adsorption results, all the adsorption data were analyzed three times and the results were averaged. 2.5. Recycling experiment of PR-MNPs to Hg(II) ions

The PR-MNPs were fabricated using previously reported onestep process [39]. In a typical procedure, 100 ml of an aqueous solution containing 7.5 mM of rhodanine was heated to 90 °C. After the solution was heated, iron chloride (6.2 mM) and sodium borohydride (26 mM) were sequentially injected into the rhodanine dissolved water. After then, the reactor was sealed up and vigorously stirred. The iron ions became magnetic c-Fe2O3 nanoparticles, and then the rhodanine monomer was polymerized on the surface of magnetic nanoparticles with vigorous stirring for 10 h. After polymerization, the synthesized PR-MNPs were obtained by external magnetic field and washed several times with more than 100 mL of distilled water to remove the residual reagents. 2.3. Effects of pH and initial metal ion concentration on adsorption of Hg(II) ions In this work, the all the tested PR-MNPs were the fresh one except for the recycling test. Five milligrams of fabricated PR-MNPs was dispersed in 10 mL of mercury nitrate solution at serial pH values (2.0–8.0). The pH value was adjusted with 0.1 M of nitric acid and 0.1 M of ammonia solution. The prepared samples were shaken for 12 h at 350 rpm with mechanical shaker (VS-101 Green Sseriker, Vision Scientific Co.). The residual concentration of the mercury ion was measured by inductively coupled plasma (ICP) analysis after the adsorbent was recovered from the solution by an external magnetic field. Furthermore, the relationship between adsorption capacity and initial concentration of metal ion was also investigated. Five milligrams of magnetic polyrhodanine nanoparticles was added into each flask containing 10 mL of Hg(II) ion solutions with various initial metal ion concentrations (from 1 mg/L to 80 mg/L). All the flasks were shaken at 350 rpm in a rotary shaker for 12 h. The adsorbed amount of metal ions onto the PR-MNPs was calculated according to the following equations:



C0  C  100ð%Þ C0

ð1Þ

ðC 0  C e Þ  V W

ð2Þ

Qe ¼

where q is the adsorptivity (%), Qe is the equilibrium adsorption capacity of adsorbent in mg (metal)/g (adsorbent), Co and C is the

Five milligrams of the PR-MNPs was added into 10 mL of Hg(II) ion solution (80 mg/L) at a pH value of 4.0. Then, prepared samples were shaken at 350 rpm using a mechanical shaker at 25 °C. After 4 h, the PR-MNPs were harvested from the solution using an external magnetic field and the residual metal ion concentration was determined using inductively coupled plasma (ICP) analysis. In order to regenerate the metal-binding property of the PR-MNPs, the recovered PR-MNPs were dispersed to 10 mL of 0.1 M hydrochloric acid solution and shaken for 3 h. Then, the PR-MNPs were collected using an external magnetic field and washed repeatedly with excessive amount of deionized water (pH 7) to neutralize the acidic condition. After drying in a vacuum oven at room temperature, the PR-MNPs were re-added into the fresh mercury ion solution to investigate the potential to be a recyclable heavy metal ion removal agent. The recovery rate was calculated according to the following equation:

Re ¼

Qr  100ð%Þ Qo

ð3Þ

where Re is the recovery rate (%), Qo is the uptake of metal ions by the fresh PR-MNPs in mg (metal)/g (adsorbent), and Qr is the uptake of metal ions after recovery procedure in mg(metal)/g (adsorbent). 2.6. Characterization The image of transmission electron microscope (TEM) was obtained with a JEOL JEM-200CX (JEOL, Japan). Acceleration voltage for TEM was 200 kV. The X-ray diffraction (XRD) patterns of the PR-MNPs were obtained with a D5005 powder X-ray diffractometry (Bruker, Germany) with Cu Ka radiation (k = 1.5406 Å) at a scanning rate of 1 deg/min. The X-ray photoelectron spectra were performed with an AXIS-His X-ray photoelectron spectroscopy (XPS) analyzer (KRATOS). The magnetic properties of the PR-MNPs were measured using a SQUID magnetometer at 300 K between 20 and +20 kOe (Quantum Design MPMS5). The UV–vis spectra were taken with a Perkin-Elmer Lambda-20 spectrometer (Perkin-Elmer, USA). Fourier transform infrared (FT-IR) spectra were recorded on a Bomem MB 100 spectrometer (Quebec, Canada) in the absorption modes at a resolution of 4 cm1 and 32 scans. ICP

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analysis was performed with an ICPS-7500 (Shimadzu, Japan), and the BET surface area was measured with an ASAP 2000 (Micromeritics, USA). Thermogravimetric analysis was carried out in nitrogen gas flow using a TGA 2050 analyzer (TA Instruments).

2.7. Confidence interval Unless clearly defined, the 95% confidence interval for data values is reported in all cases.

3. Results and discussions 3.1. Fabrication and characterization of PR-MNPs The overall synthetic procedure of the PR-MNPs is illustrated in Fig. 1. When iron chloride was added into the rhodanine aqueous solution, the rhodanine molecules coordinated with Fe ions. After the injection of sodium borohydride, magnetic nanoparticles were instantaneously formed with coordinated-rhodanine monomers as a stabilizer. As the reaction proceeded, the Fe ions were redissolved out from the surface of magnetic nanoparticles [39]. Then, the Fe ions induced the oxidation of rhodanine monomers, leading to the chemical oxidation polymerization. In the chemical oxidation polymerization, Fe ion accepts an electron from rhodanine monomer by triggering the polymerization of rhodanine (Supporting information). At the end of the polymerization, the PR-MNPs were obtained after washing with distilled water. TEM images show that the PR-MNPs are synthesized with an average diameter of ca. 10 nm (Fig. 2). The inset image of Fig. 2 clearly exhibits the core–shell morphology of the polyrhodanineencapsulated magnetic nanoparticle. Further characterization was performed in order to obtain detailed information of the characteristics of the core magnetic nanoparticles. The X-ray diffraction pattern of fabricated magnetic polymer nanoparticles matched well with that of standard c-Fe2O3 [46]. The X-ray photoelectron spectrum displayed that the Fe 2p of synthesized PR-MNPs had the binding energy of 710.75 eV with the full-width at half-maximum (FWHM) of 3.75 eV which accorded with the reported value of typical c-Fe2O3 [47]. Therefore, it could be considered that the core part of the PR-MNPs consists of maghemite (c-Fe2O3).

Fig. 2. TEM images of the polyrhodanine-encapsulated magnetic nanoparticles (the arrow of inset TEM image indicates the polyrhodanine shell part of magnetic polymer nanoparticles).

The polyrhodanine shell was analyzed with FT-IR and UV–vis spectroscopic analysis. In the FT-IR spectrum of PR-MNPs, polymer peaks were observed at 1650, 1500, and 1180 cm1, which were attributed to the [email protected], [email protected], and CAO stretching vibration of the polyrhodanine, respectively [30,31]. In the UV–Vis spectrum, broad p–p conjugated polyrhodanine backbone appeared at 540 nm in the PR-MNPs [30,31]. Consequently, it could be concluded that the polymerization of rhodanine monomer successfully proceeded on the surface of magnetic nanoparticles. In the room temperature magnetization curve as a function of applied magnetic field, it was shown that the as-prepared PRMNPs had the saturation magnetization (Ms) of ca. 28 emu/g which was lower than that of bulk c-Fe2O3 (Ms = 76 emu/g) due to their nano-scale size [48]. The coercivity (Hc) of the fabricated nanoparticles was about 16 Oe. The results presented that the PR-MNPs had typical ferromagnetic behavior [49]. The BET surface area of the PR-MNPs was measured to be 94.65 m2/g. In addition, thermogravimetric analysis was performed for quantitative analysis of the magnetic core content in the nanoparticles. As a result, it was verified that the PR-MNPs were consisted of ca. 80 wt.% maghemite.

Fig. 1. Schematic illustration of the fabrication process of polyrhodanine-encapsulated magnetic nanoparticles.

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3.2. Effect of solution pH Heavy metal ion uptake capacity of adsorbents is affected by several factors such as surface properties of adsorbent, temperature, contact time, solution pH, and initial metal ion concentration. Among them, in this paper, some representative factors were inspected for the adsorption behavior of magnetic polyrhodanine nanoparticles. First, the effect of solution pH on the adsorption performance of the PR-MNPs was evaluated using Hg(II) ion as a testing heavy metal ion (Fig. 3). It can be found that the adsorption capacity (obtained from (2))) increases sharply with a pH rise from 2.0 to 4.0 but increases slightly with a pH rise from 4.0 to 8.0. It means that the concentration of H+ ion significantly affects the adsorption behavior of PR-MNPs. At a low pH value, the abundant competitive H+ ions dominantly occupied the binding sites of the Hg(II) ions, leading to protonated functional groups. As the number of protonated metal-binding groups of the polyrhodanine gradually grew, the number of adsorption sites available for Hg(II) ions decreased. As a result, the Hg(II) ions adsorption performance of the PR-MNPs was greatly weakened as decreasing the solution pH. On the other hand, as the pH value increased, the adsorption capacity of Hg(II) ion onto the PR-MNPs was enhanced because the protonated groups were deprotonated. In addition, the surface charge of the PR-MNPs also affects to the Hg(II) ions removal performance. As the pH value increases, the surface charge the sample decreases (Supporting information). The negative surface charge enhances the affinity to positively charged metal ions. The increment rate of adsorption capacity diminished in the pH range of 4.0 to 8.0. These results could be interpreted that the Hg(II) ion adsorption activity of the fabricated nanoparticles slowly approached a saturation state as the pH increases. Therefore, the solution pH of 4.0 could be an optimum value for the Hg(II) ion removal application of the PR-MNPs. 3.3. Effect of initial Hg(II) concentration with isothermal models The Hg(II) ion removal performance of the PR-MNPs was evaluated as a function of the initial Hg(II) ion concentration (from 1.3 mg/L to 80 mg/L) at a pH value of 4.0. In this study, the adsorption time was fixed at 12 h because the adsorption of Hg(II) ions onto the magnetic polyrhodanine nanoparticles sufficiently reached an equilibrium state for 12 h based on the following contact time dependency test. As illustrated in Fig. 4, the Hg(II) ion adsorption capacity depended on the initial Hg(II) ion concentration. Notably,

Fig. 4. Effect of initial Hg(II) ion concentration on adsorption capacity of the PRMNPs. The pH value was adjusted as 4.0 and 5.0 mg of the PR-MNPs were contacted with Hg(II) ions for 12 h at 25 °C.

when the initial mercury ion concentration was lower than ca. 45 mg/L, the Hg(II) adsorption capacity relatively rapidly increased with increasing of the initial mercury concentration. In the range of these concentrations, Hg(II) ions could bind to the abundant adsorption sites on the surface of the PR-MNPs, leading to the distinctively increased adsorptivity of the PR-MNPs. Above the 45 mg/L of initial Hg(II) ion concentration, the rate of increment of adsorption capacity became gradually slow during the initial Hg(II) ion concentration increase, whereas the adsorptivity (obtained from Eq. (1)) exhibited the opposite behavior. The highest Hg(II) ion adsorptivity was 94.5% at the initial mercury ion concentration of around 1.3 mg/L. Two different isotherm models (linearized Langmuir and Freundlich), the universally applied models for isotherm adsorption analysis, were applied to the obtained adsorption data. In the Langmuir model, it is assumed that all the adsorption sites of the adsorbent have an identical binding energy and each site binds to only a single adsorbate [50]. The Langmuir isotherm is given as:

qe ¼

qm bC e 1 þ bC e

ð4Þ

The linearized Langmuir isotherm is given as:

Ce 1 Ce ¼ þ qe qm b qm

ð5Þ

where qe is the equilibrium adsorption capacity of adsorbent in mg (metal)/g (adsorbent), Ce is the equilibrium concentration of metal ions in mg/L, qm is the maximum amount of metal adsorbed in mg (metal)/g (adsorbent), and b is the constant that refers to the bonding energy of adsorption in L/mg. On the contrary, the Freundlich model is based on a reversible heterogeneous adsorption; heterogeneity of binding energies of adsorption sites [51]. The Freundlich isotherm is given as:

qe ¼ K f C e1=n

ð6Þ

The linearized Freundlich isotherm is given as:

log qe ¼ log K f þ

Fig. 3. Effect of solution pH on Hg(II) ion adsorption capacity of the PR-MNPs. In each case, 5.0 mg of PR-MNPs was dispersed in the 10 mL of Hg(II) ion solution (initial Hg(II) ion concentration was 80 mg/L) and the adsorption was proceeded for 12 h at 25 °C. The calculated standard deviation was less than 5%, and the 89.44% of the Hg(II) ions were removed at a pH value of 7.98.

1 log C e n

ð7Þ

where qe is the equilibrium adsorption capacity of the adsorbent in mg (metal)/g (adsorbent), Ce is the equilibrium concentration of heavy metal ions in mg/L, Kf is the constant related to the adsorption capacity of the adsorbent in mg/L, and n is the constant related to the adsorption intensity. The quantitative relationship between initial Hg(II) ion concentration and the adsorption capacity is analyzed with two different isotherm models (Fig. 5). The calculated

J. Song et al. / Journal of Colloid and Interface Science 359 (2011) 505–511

Fig. 5. Adsorption isotherm of Hg(II) ion onto the PR-MNPs (a) Langmuir model and (b) Freundlich model.

Table 1 Adsorption parameters of the Langmuir and Freundlich isotherm models for the adsorption of Hg(II) ion onto the PR-MNPs. Langmuir model

Freundlich model

qm (mg/g)

b (l/mg)

R2

F-test

Kf

n

R2

F-test

179

0.165

0.974

0.98

3.74

1.55

0.992

0.99

correlation coefficients (b, qm, n, and Kf) and linear regression coefficient (R2) values for each Langmuir and Freundlich model are shown in Table 1. Comparing the R2 value and F-test result of both models, it is concluded the experimental data are more fitted to the Freundlich model, which suggests the heterogeneous metal ion adsorption activity. It may result from the different adsorption sites (oxygen, nitrogen, and sulfur groups) of polyrhodanine that have different metal-binding energies. 3.4. Adsorption kinetics of metal ion adsorption process Kinetic analysis of adsorption process is very important for the design of adsorbents because the kinetics provide essential information on the adsorption mechanism and the metal ion uptake rate. Fig. 6a displays the time dependence of heavy metal ion adsorption capacity onto the PR-MNPs. For the kinetic test, Hg(II), Cd(II), Mn(II), and Cr(III) ions were selected as adsorbates. The adsorption capacities of all four metal ions reach an equilibrium state within 2 h. As a result of the kinetic tests, the obtained adsorption capacities for the four metal ions were in the decreasing order of Hg(II) >> Cr(III) > Cd(II)  Mn(II) in this study based on the

509

Fig. 6. (a) Time dependence of adsorption capacity of Hg(II), Cd(II), Cr(III), Mn(II) ions onto the PR-MNPs and (b) equilibrium adsorption amount of heavy metal ions in moles. The test was proceeded at a pH value of 4.0 and 25 °C. The initial concentration of the each metal ion was 80 mg/L.

molar amount of metal ions (Fig. 6b). According to the HSAB theory, Hg(II) and Cd(II) ions are classified as Lewis soft acids. Meanwhile, the sulfur groups of polyrhodanine are classed as Lewis soft bases which have strong affinity to Lewis soft acids [38]. Based on the HSAB theory, it could be considered that the Hg(II) and Cd(II) ions predominantly bound with sulfur groups of the polyrhodanine. On the contrary, the polyrhodanine shell of the fabricated nanoparticles also has oxygen and quaternary nitrogen groups regarded as Lewis hard bases; they have affinity to Lewis hard acids like Mn(II) and Cr(III) ions [38]. Although the reason of metal-binding priority is not clearly proved, it could be suggested that different preferable binding sites of the four metal ions caused the different uptake capability onto the magnetic polyrhodanine nanoparticles. In addition, it is revealed that the Fe2O3, the core part of the PR-MNPs, also has certain degree of metal-binding property (Supporting information). Therefore, the partially exposed Fe2O3 part can affect the metal-binding property. [52] The disparity in metal ion radius, interaction energy, and oxidation states of the heavy metal ions could also affect the different adsorption capability [53]. In order to investigate the mechanisms of metal adsorption process, the linearized equations of pseudo-first-order and pseudosecond-order kinetic models were applied and the results were shown in Fig. 7. The pseudo-first-order kinetic model assumes that the binding is originated from physical adsorption and the equation is given as [54]:

logðqe  qt Þ ¼ log qe 

K1 t 2:303

ð8Þ

where qe and qt are the amount of heavy metal ions adsorbed on the adsorbent in mg (metal)/g (adsorbent) at equilibrium and at time t,

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Fig. 7. (a) Pseudo-first-order and (b) pseudo-second-order kinetic adsorption of Hg(II), Cd(II), Cr(III), and Mn(II) ions onto the PR-MNPs. The test was performed at a pH value of 4.0 and 25 °C. The initial concentration of the each metal ion was 80 mg/L.

respectively. The K1 is the constant of first-order kinetics in min1. On the other hand, the pseudo-second-order kinetic model is based on chemical adsorption (chemisorption) and the equation is given as [54]:

t 1 t ¼ þ qt K 2 q2e qe

ð9Þ

where qe and qt are the amount of heavy metal ions adsorbed on the adsorbent in mg (metal)/g (adsorbent) at equilibrium and at time t, respectively. The K2 is the rate constant of second-order kinetics in g/(mg min). The values of qe, K1, and K2 were experimentally determined from the Eqs. (8) and (9). The obtained parameters from two kinetic models are listed in Table 2. Obviously, the metal ions adsorption process of the PR-MNPs can be well described by the pseudo-second-order model, which assumes that the determining adsorption rate depends on chemical adsorption [54]. Judging from these data, it can be suggested that the adsorption behavior of the heavy metal ions onto the PR-MNPs is promoted by a chemical pro-

Table 2 Kinetic adsorption parameters obtained using pseudo-first-order and pseudo-secondorder models. Metal ions

Hg(II) Cd(II) Cr(III) Mn(II)

Pseudo-first-order

Pseudo-second-order

K1

R2

F-test

K2 (104)

qe

R2

F-test

0.0042 0.0038 0.0052 0.0036

0.976 0.881 0.816 0.583

0.971 0.065 0.875 0.699

9.53 84.6 93.3 104

133 19.4 9.91 8.84

0.999 0.999 0.999 0.999

0.999 0.999 0.996 0.999

Fig. 8. (a) Photographs of the dispersed and harvested PR-MNPs and (b) the adsorption efficiency of Hg(II) in adsorption–desorption cycles by the PR-MNPs. The initial Hg(II) ion concentration was 80 mg/L and pH value was 6.0. Five milligrams of the PR-MNPs was contacted with Hg(II) ions for 4 h at 25 °C.

cess involving valence forces through sharing or exchange of electrons. 3.5. Recycling study of the PR-MNPs For the recycling experiment, the Hg(II) ion was selected as adsorbate because the Hg(II) ion exhibited highest adsorption capacity among the tested metal ions in our experimental condition. After the typical adsorption test, the PR-MNPs were harvested using external magnetic field. Then, the harvested PR-MNPs were sequentially treated with a 0.1 M hydrochloric acid solution and deionized water to regenerate metal ion binding ability. Through these simple regeneration processes, the bound Hg(II) ions were washed out and the PR-MNPs recovered the metal ion binding performance. The variation of adsorption efficiency with respect to the recycle number is displayed in Fig. 8b. The metal ion binding capability of the PR-MNPs remained above 96% after 5 times of recycling test. The result of the recovery test indicates that the PRMNPs can be used as recyclable adsorbents for heavy metal ions removal. 4. Conclusion The polyrhodanine-encapsulated magnetic nanoparticles were synthesized via one-step chemical oxidation polymerization. Based on the metal-binding properties of the polyrhodanine, the heavy metal ions adsorption activities of fabricated magnetic polymer nanoparticles were investigated with various experimental conditions. As a result, it was verified that the uptake capacity of heavy metal ions increased as the solution pH or initial metal ion concen-

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tration increased. The Freundlich isotherm model fits the Hg(II) ion adsorption behavior of the PR-MNPs better than the Langmuir model. In addition, the adsorption kinetic study suggests that the adsorption of metal ions onto the PR-MNPs was mainly performed through a chemical binding process. The recycling test presented that the PR-MNPs could be easily harvested and repeatedly used for the removal of heavy metal ions owing to their magnetic and regeneration properties, respectively. Judging from these results, it is anticipated that the easily recyclable polyrhodanine-coated magnetic nanoparticles can be applied to the heavy metal removal from contaminated water. Acknowledgments This research was supported by World Class University (WCU) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-10013). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.04.034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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